Cold Starting probelms

Hello to all

Can anyone advise on likely causes for non start of the 2.0 HDi

I have replaced all glow plugs with new and it was working fine for
several
weeks but then would not start.

It seems that if the air is very damp it won't start.

I have an electric fan heater which I put under the bonnet for about
20mins - it will start after this.

I don't think that the heater imparts enough heat to the engine to make
the
head warm enough to allow the engine to start - could it be that the
heater
dries out the air in the intake and this solves the problem?

The external temp sensor re****ts temp OK - is there another sensor looking
at cold/humidity etc that could cause this?

Next stop is a new preheat control box but I am not convinced this will
resolve the problem

Any help gratefully received.

Many thanks

Andrew